Monkeying about with patterns of sleep

Press, 30 June 1982, Page 20

Monkeying about with patterns of sleep

From the “Economist,” London;

A new study of the brain’s activity during sleep deepens the mystery about the functions sleep pierforms. Researchers from America’s National Institute of Mental Health (N.1.M.H.) measured the brain metabolism of rhesus monkeys during sleep by analysing the brain’s uptake of

glucose. The results, published in “Nature,” show an average reduction in activity (compared with wakefulness) of 30 per cent. The scientists examined activity in 75 separate regions of the brain and found reductions varying from 5 per cent to 44 per cent. At first, this sounds compatible with the idea that the brain is resting during sleep. But that explanation is not entirely satisfactory. If resting is the function of sleep, you would expect the most active areas during wakefulness to be least active during sleep. That is not what the researchers found. Another possibility is that, rather than recovering from the day’s activity, the brain is simply conserving energy. (The brain accounts for a quarter of the body’s total metabolism.) In this case, however, one would expect a reduction in brain activity greater than 30 per cent. An old wives’ tale says that sleep, is the time when bodily growth takes place. This idea seemed to acquire scientific credibility when it was discovered that large amounts, of growth hormone are released from the human brain early in sleep. Again, there are difficulties with this interpretation. For instance, most species studied do not release growth hormone during sleep. ,• •< - Another theory is that brain tissue is being repaired during

sleep — or that memories are being laid down. The evidence for either hypothesis is debatable. . Alternatively, sleep may simply b<f a way of keeping an animal inactive, so that it escapes detection by predators. The trouble with that hypothesis is that it is hard to test.

Length of sleep differs enormously arpong species. The giant sloth' sleeps 20 hours a day, whereas the horse takes only two hours sleep out of every 24. The bottlenose dolphin has most unusual sleeping habits. First one half of the brain goes to sleep, then the other half takes its turn at sleeping. Some humans habitually sleep hardly at all, apparently without suffering. In expert? ments where •normal sleepers are deprived |of sleep completely for days on end, few ill effects are otaerved. One thing that does suffer is the ability to pav attention. { Humans have 4 an internal clock associates with their sleep/wake cyclj. This clock normally works bn a “day” length slightly ledger than 24 hours. | Sleep researchers have hit on a novel way of helping some insomniacs who cannot get to sleep until very late. The idea is to set the insomniac’s internal clock forward. The next day he is told to stay awake'until 6 a.m., the next day 9 a.m. and so on until he reaches the desired bed. time. He then settles down into a regular pattern of conventional sleeping. *•’, Another discovery is that, when experiments are done putting people in caves without access to a clock, they gradually stay awake — and sleep — for longer periods. One person ended up sleeping 19 hours at a

stretch and then staying awake for 31 hours. Such people are amazed when they emerge from the cave to find how many days have really passed. If the function of sleep is a mystery, , the function of ’ dreams is even more of one. The N-IM.H. researchers studied brain metabolism during the periods of ordinary sleep. They would dearly like to oo similar experiments on ' so-called? ‘rapid-eye-movement (R.E.M.) sleep which is associ- : ated with dreams. ,

Unfortunately, R.E.M. periods in monkeys are typically 6-8 minutes, whereas an experiment has to last 30 minutes to get useful information about brain metabolism. Although humans do have 30minute R.E.M. periods, the usual experiment involves killing the subject and slicing up its brain. This can be avoided by observing glucose metabolism on a machine called a positron-emission tomograph (P.E.T.). The snag, is that P.E.T.S are very expensive and

it is hard to obtain time on them. - .

Presumably, there’ is a reason why ? humans have longer dreaming periods than monkeys. But so far scientists have not demonstrated what the benefits of dreams might be. • \ - .

In fact, it has been found that the effectiveness of some drugs used to treat severecases of depression is due to the fact that these drugs suppress the amount the patients dream.